The present invention relates generally to multi-element seal assemblies for effecting a seal between relatively movable parts and, more particularly, to an improved, low pressure sealing element for use with such multi-element seal assemblies.
Multi-element seal assemblies, particularly such assemblies for use in providing a seal between relatively reciprocating surfaces, such as pistons and surrounding cylinders and/or piston rods and surrounding housings, are generally well known in the art. Such multi-element seal assemblies are disclosed in U.S. Pat. Nos. 4,421,330; 4,231,578; 4,268,045; and 4,566,702. These prior art seal assemblies generally include at least two interacting sealing elements: a low pressure sealing element formed of a relatively resilient, deformable sealing material, such as synthetic or natural rubber, and a high pressure sealing element formed of a relatively rigid sealing material, such as PTFE. Both the low pressure sealing element and the high pressure sealing element are generally annular and are typically installed in an annular sealing groove within one of the reciprocating surfaces with the groove opening facing the other surface. Typically, the low pressure sealing element seats against the bottom or base of the groove, with the high pressure sealing element positioned between at least a portion of the low pressure sealing element and the other reciprocating surface. As shown in the above-referenced patents, at least a portion of the low pressure sealing element, sometimes called the wiper or sealing lip, extends beyond the sealing groove. During low pressure operation (below a predetermined pressure limit), the wiper or sealing lip engages the other reciprocating surface to provide primary sealing. At higher pressures (at or above the predetermined pressure limit), the low pressure sealing element is forced by the pressure into the sealing groove, thereby partially withdrawing the wiper from engagement with the other reciprocating surface and correspondingly urging the high pressure sealing element into sealing engagement with the other reciprocating surface. In general, the pressure limit is determined by the specific material used to form the low pressure sealing element.
While such seal assemblies generally provide effective sealing at both low and high pressures, such seal assemblies, due to their structure, sometimes provide unnecessary friction upon the other reciprocating surface during high pressure operation. Such unnecessary friction, in some cases, is caused by the wiper of the low pressure sealing element not being effectively withdrawn into the groove, and in other cases may be due to the low pressure sealing element putting excess outward pressure upon the high pressure sealing element. In this manner, a portion of the sealing lip or wiper continues to engage the other reciprocating surface during high pressure operation and/or the high pressure sealing element tightly engages the other reciprocating surface, resulting in additional friction, as well as decreased life of the low pressure sealing element, due to premature wear. In addition, it has been determined that such prior art seal assemblies are often inefficient during the transition from high pressure operation to low pressure operation. More specifically, it has been determined that with such prior art seal assemblies, there is a delay in the complete reengagement of the low pressure sealing element with the other reciprocating surface as the operating pressure decreases. Such a delay or inconsistency may result in an unacceptable loss of fluid between the reciprocating surfaces.
The present invention comprises an improved seal assembly in which the low pressure sealing element is at least partially withdrawn from engagement with the other reciprocating surface during high pressure operation to provide decreased friction and enhanced life for the low pressure sealing element. The present invention also provides a low pressure sealing element which is more efficient and provides more effective sealing during the transition from high pressure operation to low pressure operation.